A casting assembly typically consists of a pouring cup, a gating system (including downsprues, choke, and runner), risers, molds, cores, and other components. To produce a metal casting, metal is poured into the pouring cup of the casting assembly and passes through the gating system to the mold and/or core assembly where it cools and solidifies. The metal part is then removed by separating it from the core and/or mold assembly.
The molds and/or cores used in the casting assembly are typically made of sand and a binder, often by the no-bake or cold-box process. The sand is mixed with a chemical binder and typically cured in the presence of a liquid or vaporous catalyst after it is shaped.
Risers are cavities in which excess molten metal flows. The excess molten metal is needed to compensate for contractions or shrinkage of metal, which occur during the casting process. Metal from the riser fills such voids created in the casting when metal from the casting contracts. The metal from the riser must remain in a liquid state for a longer period of time, so it can provide molten metal to the casting as it cools and solidifies. Thus, it is advantageous to keep the molten metal in the riser hot as long as possible.
Heat loss from the riser occurs by convection to the cooler surroundings and through radiation to the cooler atmosphere. Because of this problem with heat loss associated with using open risers, closed risers that are surrounded by and covered with sleeve material, are sometimes used instead of open risers. One problem associated with the use of closed risers is that the operator cannot see when the riser cavity is full by visual inspections. In addition, closed risers do not provide venting of mold gasses to the atmosphere during pouring. These conditions can result in over-filling of the mold, metal spillage, and resulting safety hazards.
Because of the problems associated with using closed risers, open risers are sometimes preferred. When an open riser is used, the operator can visually inspect the riser cavity and determine when the level of molten metal in the riser cavity is appropriate. After the appropriate level is reached, in order to prevent heat loss from an open riser, the top of the riser cavity is covered with a hot-topping, e.g. a granular material, a powder, rice hulls, a blanket (see U.S. Pat. No. 3,876,420), and solid covers (graphite) having an insulating properties, exothermic properties, or both, within a relatively short period of time to prevent excessive heat loss. When an open riser is used, typically an extra person is needed to inspect the riser cavity and apply the topping following pouring.
If the hot-topping is a granular or powder material, it often spills across the top of the casting assembly onto the floor of the foundry. Because there is often spillage or misapplication, it is normal practice to apply much more than the optimum amount that is necessary. Additionally, when powdered materials are used, the powdered materials can miss the top of the riser and spill onto the casting assembly where it can eventually get mixed into the molding sand and consequently cause casting defects.
If blankets are placed on top of the riser, before the riser is filled with metal, the metal pourer is not able to see the metal fill the riser and the molten metal could overflow and spill onto the floor. If blankets are placed on the riser after the appropriate level of molten metal is reached, an extra person is usually required to inspect the riser cavity and to place the blanket on top of the open riser while the metal pourer moves on to pour the next mold. Furthermore, the metal in the riser is open to the atmosphere during the time between filling and when the blanket is applied, which results in heat loss. The longer the delay before the cover is placed over the cavity, the more heat is lost and the effectiveness of the riser is reduced.
WO 2004/000485 teaches that this problem can be solved if a riser insert, which is slightly smaller than the internal cross section of the riser, is inserted into the riser cavity before the metal is poured. Although the advantages of using the riser insert were realized in actual foundry applications, there was a problem that arose in using it in some applications. It was found that the riser inserts sometimes were not held in place by the riser and floated out of the riser cavity when the metal was poured, thus losing their effectiveness.
All citations referred to under this description of the “Related Art” and in the “Detailed Description of the Invention” are expressly incorporated by reference.